The Spiderweb proto-cluster is being magnetized by its central radio jet

Craig S. Anderson, Christopher L. Carilli, Paolo Tozzi, G. K. Miley, S. Borgani, Tracy Clarke, L. Di Mascolo, Ang Liu, Tony Mroczkowski, Maurilio Pannella, L Pentericci, H. J. A. Rottgering, A. Saro

Submitted on 7 July 2022


We present deep broadband radio polarization observations of the Spiderweb radio galaxy (J1140-2629) in a galaxy proto-cluster at z=2.16. These yield the most detailed polarimetric maps yet made of a high redshift radio galaxy. The intrinsic polarization angles and Faraday Rotation Measures (RMs) reveal coherent magnetic fields spanning the 60 kpc length of the jets, while 50% fractional polarizations indicate these fields are well-ordered. Source-frame absolute RM values of 1,000 rad/m/m are typical, and values up to 11,100 rad/m/m are observed. The Faraday-rotating gas cannot be well-mixed with the synchrotron-emitting gas, or stronger-than-observed depolarization would occur. Nevertheless, an observed spatial coincidence between a localized absolute RM enhancement of 1,100 rad/m/m, a bright knot of Lyα emission, and a deviation of the radio jet provide direct evidence for vigorous jet-gas interaction. We detect a large-scale RM gradient totaling 1,000s rad/m/m across the width of the jet, suggesting a net clockwise (as viewed from the AGN) toroidal magnetic field component exists at 10s-of-kpc-scales, which we speculate may be associated with the operation of a Poynting-Robertson cosmic battery. We conclude the RMs are mainly generated in a sheath of hot gas around the radio jet, rather than the ambient foreground proto-cluster gas. The estimated magnetic field strength decreases by successive orders-of-magnitude going from the jet hotspots (90 μG) to the jet sheath (10 μG) to the ambient intracluster medium (1 μG). Synthesizing our results, we propose that the Spiderweb radio galaxy is actively magnetizing its surrounding proto-cluster environment, with possible implications for theories of the origin and evolution of cosmic magnetic fields.


Comment: 12 pages, 7 figures, 2 tables. Accepted for publication in ApJ

Subjects: Astrophysics - Astrophysics of Galaxies; Astrophysics - Cosmology and Nongalactic Astrophysics; Astrophysics - High Energy Astrophysical Phenomena